Children who are born deaf gain access to the auditory world through the use [unreadable] of cochlear implants (CIs), which provide electrical stimulation directly to the auditory nerve. Even though most children can perform quite well with a single implant, with better results associated with earlier implantation, [unreadable] there has been a recent clinical trend to provide patients who are deaf with bilateral CIs. Although this practice is becoming widely accepted for older patients, it is still under debate for infants. One reason for this debate is [unreadable] that the benefit of providing bilateral CIs to infants has not been systematically studied. In older children and adults, bilateral CIs have been shown to improve both spatial hearing and speech recognition in noise, [unreadable] consistent with the idea that binaural hearing can improve performance on various listening tasks. These binaural benefits have yet to be shown in infants who are born deaf and use CIs. Thus, there is a need to document the [unreadable] potential benefits of bilateral CIs in very young children, especially at a time when language learning and brain plasticity are greatest. To accomplish this, we will measure two auditory-dependent processes, auditory word learning [unreadable] and spatial hearing, in toddlers (under the age of three years) who use either a single CI or bilateral CIs. A combination of psychophysical and behavioral techniques, which are well-established for use in typically-developing [unreadable] toddlers, will be utilized in this proposal. In the experiments designed to test auditory word learning, word recognition ability in quiet and in the presence of noise will be assessed by using a looking-preference paradigm, and [unreadable] novel word learning will be evaluated using a word-object association task. In the experiments designed to assess spatial hearing, an observer-based method will be used to determine a toddler's minimum audible angle, which is a [unreadable] quantification of sound localization ability. The purpose of these experiments is to document the development of language and spatial hearing in very young CI users as well as to test the hypothesis that children with bilateral CIs [unreadable] will outperform children who use a single CI on these tasks. The knowledge that can be gained from this project is clinically applicable and necessary due to the current debate regarding the cost-effectiveness of providing [unreadable] bilateral CIs (versus one CI). The data generated from this study will provide insight into the development of language and auditory skills in very young children who use CIs, as well as serve to inform decisions about providing [unreadable] unilateral versus bilateral CIs to infants.[unreadable] [unreadable] [unreadable]